Engines that burn diesel fuel are the most popular type of compression ignition engines. So-called diesel engines introduce fuel at high pressure directly into the combustion chamber. Diesel engines are very efficient because this allows high compression ratios to be employed without the danger of knocking, which is the premature detonation of the fuel mixture inside the combustion chamber. Because diesel engines introduce their fuel directly into the combustion chamber, the fuel injection pressure must be greater than the pressure inside the combustion chamber when the fuel is being introduced, and, for liquid fuels the pressure must be significantly higher so that the fuel is atomized for efficient combustion.
Diesel engines are favored by industry because they are proven performers that are known to give operators the best combination of power, performance, efficiency and reliability. For example, diesel engines are generally much less expensive to operate compared to gasoline fueled spark-ignited engines, especially in high-use applications where a lot of fuel is consumed. However, a disadvantage of diesel engines is that they can produce more pollution, such as particulate matter (soot) and NOx, which are subject to increasingly stringent regulations that require such emissions to be progressively reduced over time. To comply with such regulations, engine manufacturers are developing catalytic converters and other after treatment devices to remove pollutants from the exhaust stream. Improvements to the fuel are also being introduced, for example to reduce the amount of sulfur in the fuel, to prevent sulfur from de-activating catalysts and to reduce air pollution. Research is being conducted to improve combustion efficiency to reduce engine emissions, for example by making refinements to engine control strategies. However, most of these approaches add to the capital cost of the engine and/or the operating costs.
Recent developments have been directed to substituting some of the diesel fuel with cleaner burning gaseous fuels such as, for example, natural gas, pure methane, butane, propane, hydrogen, and blends thereof. However, in this disclosure “gaseous fuel” is defined more broadly than these examples. Gaseous fuel is defined herein as any combustible fuel that is in the gaseous phase at atmospheric pressure and ambient temperature. Since gaseous fuels typically do not auto-ignite at the same temperature and pressure as diesel fuel, a small amount of liquid fuel can be introduced into the combustion chamber to auto-ignite and trigger the ignition of the gaseous fuel. One approach for consuming gaseous fuel on board a vehicle involves introducing the gaseous fuel into the engine's intake air manifold at relatively low pressures. In this approach, since the gaseous fuel is inducted into the intake air stream, the supply pressure of gaseous fuel is not a limiting factor in determining when gaseous fuel can be introduced. The liquid fuel injection valve for introducing the diesel fuel into the combustion chamber can use the same orifice geometry as a conventional diesel valve, and engines with this design can operate with only diesel fuel if the gaseous fuel is not available. This can be an advantage if a vehicle powered by such an engine operates on a route where it can not replenish its supply of gaseous fuel, because if the vehicle runs out of gaseous fuel it can continue to operate using diesel fuel only. However, this feature can be viewed as a disadvantage by regulators and government agencies who offer subsidies for engines fuelled with cleaner burning gaseous fuels, because with this technology there is no easy way to ensure that an operator who applies for such a subsidy will actually fuel the subsidized engine with a gaseous fuel and engine emissions can be much higher when the engine is fuelled with only diesel fuel, compared to when the engine is fuelled with gaseous fuel and only pilot quantities of diesel fuel.
In addition, dual fuel engines that introduce fuel into the intake air manifold or intake port have been unable to match the performance and efficiency of conventional diesel engines. There are a number of factors that contribute to this shortcoming. Dual fuel engines that introduce the gaseous fuel into the intake air stream normally must be made with a lower compression ratio because the gaseous fuel can pre-mix with the air earlier in the compression stroke, introducing the potential for engine knock, which is the premature detonation of fuel in the combustion chamber. To prevent engine knock, such engines must either limit the amount of gaseous fuel that can be introduced into the combustion chamber, or reduce the engine's compression ratio. A lower compression ratio results in an engine that can not match the performance and efficiency of an engine that has the same compression ratio as a diesel engine. Another disadvantage of introducing the gaseous fuel into the intake air stream is that by occupying space in the intake air manifold, the fuel can reduce the mass of air that can flow into the combustion chamber.
In another approach to substituting gaseous fuel for most of the diesel fuel, it is possible to substantially match the performance and efficiency of a conventional diesel engine by injecting a high-pressure gaseous fuel directly into the combustion chamber. With this approach the timing for introducing the fuel into the combustion chamber can be controlled and the compression ratio can be kept the same as a conventional diesel engine. However, a problem with directly injecting both fuels into the combustion chamber is that both fuels need to be supplied at a pressure that is high enough to overcome the in-cylinder pressure, while also introducing a fuel spray that penetrates into the combustion chamber space to mix with the air. In the case of liquid fuels, the injection pressure should be high enough to atomize the fuel so that it can be efficiently combusted inside the combustion chamber. In the case of gaseous fuels, as disclosed in co-owned Canadian patent application 2,463,791, filed Apr. 7, 2004, it is desirable for the injection pressure of the gaseous fuel to be high enough to cause the gaseous fuel to expand into the combustion chamber space at supersonic velocities.
Because liquid fuels are substantially incompressible fluids, a liquid fuel pump can raise the liquid fuel pressure to the desired injection pressure almost instantly. However, because a gaseous fuel is a compressible fluid, there can be times during operation of the engine when it is not possible to supply the gaseous fuel at the desired injection pressure. For example, this can occur when the engine is starting up, or when the gaseous fuel tank is empty, or if there is a problem with the gaseous fuel pump or compressor. Without a control strategy for such occurrences, there can be times when the gaseous-fuel injection valve is operated with little or no gaseous fuel being introduced into the combustion chamber. In start-up conditions, when the gaseous-fuel supply system is being pressurized, opening the gaseous-fuel injection valve can slow the time it takes to raise gaseous fuel pressure to the normal operating level. Failing to recognize that gaseous-fuel is not being introduced into the combustion chamber with the desired injection pressure or in the desired amounts can result in poor combustion, higher particulate matter and unburned hydrocarbon exhaust emissions, and lower engine performance and efficiency, since injection timing may not be matched to the amount of fuel being introduced into the combustion chamber.
For some engines that use a gaseous fuel as the main fuel, the liquid fuel injection valves are not capable of introducing enough liquid fuel to operate the engine at full power using liquid fuel alone. For example, if the nozzle orifices are sized for delivering smaller quantities of liquid fuel with predetermined flow rates and velocities under normal operating conditions such orifices may be sized too small to introduce liquid fuel in quantities sufficient to operate the engine on only liquid fuel. For engines that are the prime mover for a vehicle, a control strategy is needed to handle the possibility of the vehicle running out of gaseous fuel and not being able to operate at full power.
Accordingly, for an engine that is incapable of operating at full power with liquid fuel only, there is a need for a control strategy that recognizes when gaseous fuel is not available, for controlling engine operation in such a situation, and for recognizing when gaseous fuel is available so the engine can resume normal operation.